The two broad, long term objectives of this proposed research are to use electrophysiological techniques to improve our understanding of the mechanisms responsible for atrial fibrillation (AF) in patients and to develop an improved basis for its treatment. The specific aims of this research plan are to study patients 1) during open heart surgery a) to map the sequence of atrial activation simultaneously in both atria i) during spontaneous AF of various types (paroxysmal, persistent, or permanent) and duration (recent onset, weeks, months, or years); ii) during overdrive pacing or programmed atrial stimulation during AF from a site or sites close to or within an area of regular rhythm of very short cycle length; iii) following cardioversion of AF to sinus rhythm; iv) during atrial pacing from selected atrial sites during sinus rhythm in patients with or without a history of AF; b) during AF, to identify and characterize areas of very regular but very short cycle length, thought to drive the atria creating fibrillatory conduction; c) to characterize the atrial effective refractory period during sinus rhythm from selected sites in both atria in patients with various forms and durations of AF, and compare that with patients with normal atria and no history of AF; and d) to obtain Fast Fourier Transform (FFT) analysis of atrial electrograms recorded during intraoperative activation mapping to identify and help characterize the presence of a region with a regular rhythm of very short cycle length; and 2) in the postoperative period to use temporary atrial epicardial wire electrodes in patients with new onset AF a) to record atrial electrograms and obtain FFT analysis of these atrial electrograms to identify and help characterize the presence of a regular rhythm of very short cycle length; and b) to perform overdrive pacing from an area demonstrating a regular rhythm of very short cycle length to try to terminate the AF. [unreadable] [unreadable]